The neuropeptide Y4 receptor (Y4) is a 375 amino acid G-protein coupled receptor (GPCR) that is ex- pressed mainly in peripheral tissues and the brain stem. In humans, Y4 belongs to a family of receptors (Y1, Y2, Y4, and Y5), that bind the ligands neuropeptide Y (NPY), polypeptide YY (PYY) and pancreatic polypeptide (PP). NPY, PYY and PP are 36 residue peptide hormones that play critical roles in regulating feeding behavior and energy homeostasis. Y4 is the only receptor subtype with low affinity for NPY and PYY and very high (picomolar) affinity for PP5. Thus, selective agonists of Y4 could be promising candidates for obesity therapeutics. In fact Obinepitide (TM-30338), a variant of PP and PYY, is currently in phase II clinical trials as a treatment for obesity. However as Obinepitide is a peptide, issues of stability and bioavailability remain. Development of non-peptidic Y4-selective ligands that bind to the endogenous binding site have failed so far. The objective of the present proposal is to identify small molecule allosteric modulators of the Y4 receptor. Allosteric modulators of GPCRs have a higher chance to be selective as allosteric binding sites tend to be evolutionary less conserved between receptor subtypes. The therapeutic potential of allosteric modulators is further increased by their ability to tune the receptor response instead of simply turning it on or off. Side effects may also be reduced for allosteric potentiators because the therapeutic only acts when the receptor is engaged by its native ligand. In preliminary work, we have adapted a Y4 functional assay for high-throughput screening (HTS) experiments that can detect agonists, antagonists and allosteric modulators simultaneously. We con- ducted a pilot screen of 2,000 compounds that yielded several hits including one, Niclosamide that displayed robust, selective allosteric potentiation with an EC50~400 nM. The proposed experiments will be used in combination with computational methods that enable virtual screening of millions of compounds. We will construct quantitative structure activity relationship (QSAR) models to create libraries focused around initial hit compounds and libraries enriched with novel chemotypes predicted to be Y4 allosteric modulators. Identification of small molecule allosteric modulators of Y4 will allow future development of pharmacological probes and can seed drug discovery programs in obesity.